To argue against Mary's perpetual virginity is to suggest something else that is greatly implausible, not to say unthinkable: that neither Mary nor her protector, Joseph, would have deemed it inappropriate to have sexual relations after the birth of God in the flesh. Leaving aside for a moment the complete uniqueness of the Incarnation of the Second Person of the Trinity, recall that it was the practice for devout Jews in the ancient world to refrain from sexual activity following any great manifestation of the Holy Spirit.
An early first-century popular rabbinical tradition (first recorded by Philo, 20 BC-AD 50) notes that Moses "separated himself" from his wife Zipporah when he returned from his encounter with God in the burning bush. Another rabbinical tradition, concerning the choosing of the elders of Israel in Numbers 7, relates that after God had worked among them, one man exclaimed, "Woe to the wives of these men!" (Hainsworth 2004).
I can remember that in my Lutheran days the mere off-handed mention of the perpetual virginity of Mary (i.e. semper virgo) would spark the longest and most tireless debate between interested Lutheran parties. Thus the topic will always hold a certain glimmering interest for me.